What is Krampusnacht?:
December 6 is the day in which parts of Germany and Bavaria celebrate Krampusnacht, which is a throwback to a pre-Christian tradition. The word Krampus means "claw", and apparently certain Alpine villiages have big parties featuring a scary clawed incubus who hangs around with Santa Claus. The Krampus costume also includes sheepskin, horns, and a switch that the incubus uses to swat children and unsuspecting young ladies. The Krampus' job is to punish those who have been bad, while Santa rewards the people on his "nice" list. There's been a resurgence in interest in Krampus over the past century or so, but it seems as though the custom goes back hundreds of years.
While the men parade around dressed as creepy demons, the women get to have some fun too, wearing masks and representing Frau Perchta, who was a Nordic goddess, who may have been an aspect of Freyja, the fertility goddess. Interestingly, in the Pennsylvania Dutch community, there's a character called Pelsnickel or Belznickel who is an awful lot like Krampus, so it appears that the tradition migrated across the water when Germans settled in America.
Krampus.com, which calls itself the official home of "Krampus, the holiday devil," calls Krampus a "dark counterpart of Saint Nicholas, the traditional European gift-bringer who visits on his holy day of December 6th... The bishop-garbed St. Nicholas rewards good kids with gifts and treats; unlike the archetypal Santa, however, St. Nicholas never punishes naughty children, parceling out this task to a ghastly helper from below."